fix: fully revert agent identity runtime wiring (#18757)

## Summary

This PR fully reverts the previously merged Agent Identity runtime
integration from the old stack:
https://github.com/openai/codex/pull/17387/changes

It removes the Codex-side task lifecycle wiring, rollout/session
persistence, feature flag plumbing, lazy `auth.json` mutation,
background task auth paths, and request callsite changes introduced by
that stack.

This leaves the repo in a clean pre-AgentIdentity integration state so
the follow-up PRs can reintroduce the pieces in smaller reviewable
layers.

## Stack

1. This PR: full revert
2. https://github.com/openai/codex/pull/18871: move Agent Identity
business logic into a crate
3. https://github.com/openai/codex/pull/18785: add explicit
AgentIdentity auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate auth callsites
through AuthProvider

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/tui/src/local_chatgpt_auth.rs

@@ -108,7 +108,6 @@ mod tests {
                 account_id: Some("workspace-1".to_string()),
             }),
             last_refresh: Some(Utc::now()),
-            agent_identity: None,
         };
         save_auth(codex_home, &auth, AuthCredentialsStoreMode::File)
             .expect("chatgpt auth should save");
@@ -155,7 +154,6 @@ mod tests {
                 openai_api_key: Some("sk-test".to_string()),
                 tokens: None,
                 last_refresh: None,
-                agent_identity: None,
             },
             AuthCredentialsStoreMode::File,
         )